Ecology: Theories and Applications 4E Chapter 1 -- Case Studies

Chapter 1: Why and How to Study Ecology
Case Studies

Drought and the Demography of Darwin’s Medium Ground Finches on Isla Daphne Major by Wendy E. Sera, Baylor University

Introduction:
One of the classic long-term ecological studies of a natural population is the body of work by Peter and Rosemary Grant and coworkers on Darwin’s finches of the Galapagos Islands. They have been studying several of the 14 species of finches found in the Galapagos Islands since 1973. The various islands in the Galapagos archipelago host different, but closely related, species. All 14 species are similar in both size and coloration, ranging in length from four to six inches, and in color from brown to black; however, there is great variation in both the size and shape of the beaks. The great range of beak morphologies among Darwin’s finches reflects the diversity of foods that they eat on the various islands. Rosemary and Peter Grant’s classic study of the evolution of beak size in Darwin’s finches is one of the first and the most elegant to document evolution in a wild population of vertebrates. In addition to this work, the Grants have studied the demographic and genetic factors that favor the long-term persistence of two species of Darwin’s finches, Geospiza scandens (the Cactus Finch) and Geospiza fortis (the Medium Ground Finch) on the Isla Daphne Major (Peter R. Grant and B. Rosemary Grant. 1992. Demography and the genetically effective sizes of two populations of Darwin’s finches. Ecology 73(3): 766–84). This case study will focus on the demography of one of these species, Geospiza fortis (the Medium Ground Finch), on Isla Daphne Major.

The population of G. fortis on Daphne Major was an ideal subject for the study of the hazards of environmental fluctuations for small populations. The populations on Daphne Major are small and are mostly restricted to the island; therefore, all population parameters (birthrate, death rate, immigration rate, emigration rate) could be determined for the entire population that occupied the island. In addition, this finch population occupied habitats that have experienced no or little disturbance by humans. Therefore, this population presented the opportunity to study a small, isolated population under natural, fluctuating environmental conditions. Such study conditions are quite rare in today’s world.

Daphne Major is a tiny, 49-ha volcanic island in the center of the Galapagos archipelago with a maximum elevation of 120 meters. Daphne Major is located about 8 km from the larger islands of Santa Cruz and Baltra. The island is seasonally arid and experiences large annual fluctuations in rainfall, and therefore in both primary and secondary production. Annual rainfall during the time of the study was highly variable and erratic as follows: Year

Year	Rainfall (mm)
1976	130
1977	20
1978	125
1979	50
1980	40
1981	55
1982	35
1983	1359
1984	40
1985	5
1986	34
1987	600
1988	0
1989	3
1990	32
1991	150
After Grant and Grant, 1992.

Note the following about rainfall: In 1975, there were late, but heavy rains followed by alternating wet and dry years. The drought in 1977 was severe, and inhibited breeding by G. fortis. Secondly, the 1977 drought caused the death of almost the entire cohort of nestlings born in 1976. Of the 338 G. fortis nestlings born in 1976, only one individual lived to breed the next year. Unlike other passerine birds, Darwin’s finches tend to exhibit strong fluctuations in annual mortality that are associated with the strong fluctuations in rainfall, which is correlated with both the food supply and finch densities.

The fates of two cohorts of G. fortis were followed from their birth until nearly every individual had died. The two cohorts were:

Nestlings born on Daphne Major in 1975, and
Nestlings born on Daphne Major in 1978.

Thus, they were able to construct cohort life tables for each group of nestlings. Breeding activities of these cohorts were followed in every detail from 1976 until 1991. The island is small enough that all nests were located and all nestlings were banded either in their year of birth, or in the year following their birth when they were mist-netted as juveniles or immatures. The following table shows the population sizes of the 1975 and 1978 cohorts of G. fortis: <I>Geospiza fortis</I> Cohorts on Isla Daphne Major

Geospiza fortis Cohorts on Isla Daphne Major (After Grant and Grant, 1992)	1975	1978
A. Banded in year of birth, Year 0 (nestlings and juveniles)	211	248
B. Survived to next year, year 1	133	138
C. Banded in next year, Year 1 (immatures)	65	20
D. Total alive in Year 1, born in Year 0	198	158
E. Starting numbers in Year 0 = (D) · (A)/(B)	314	284

These data were used to characterize the demographic structure of the population in terms of survivorship curves, fertility schedules, age distributions, net reproductive rates, as well as other parameters such as the effective population size.

A sample data set is available for this case study. The file is an Excel spreadsheet file containing four worksheets, each containing a data table as follows: Table 1

Table 1	Life table of the 1975 cohort of G. fortis.
Table 2	Life table of the 1978 cohort of G. fortis.
Table 3	Fecundity schedule for the 1975 cohort of G. fortis.
Table 4	Fecundity schedule for the 1978 cohort of G. fortis.

First, for both the 1975 (Table 1) and 1978 (Table 2) cohorts, standardize the Number Alive per 1000 (n_x1000) to allow you to compare data from the two cohorts. Use the equation below to make the necessary calculations.

Number Alive per 1000 (n_x1000) = n_x/n₁ x 1000, where n₁ is the number alive for the first age class in the life table.

To be sure that your calculations are correct, check that the top line is 1000 and the bottom line is zero.

Working from the Number Alive per 1000 (n_x1000) column, calculate the remaining life table variables (d_x, l_x, q_x, and e_x) for each age class for the 1975 cohort (Table 1) and the 1978 cohort (Table 2). Note that to calculate life expectancy (e_x), you must first calculate L_x and T_x. See section 6.1 in your textbook for directions on how to calculate these life table variables.

In words, summarize the age-specific trends in each of the life table variables (n_x1000, d_x, l_x, q_x, and e_x) for each of the cohorts. What are the differences between the two cohorts? Speculate about the cause of these differences.

To standardize your data for graphing, calculate the logarithm to the base ten (log₁₀) of each number in the Number Alive per 1000 (n_x1000) column for both the 1975 and 1978 cohorts (use the last columns in Tables 1 and 2). Again, to check your calculations, the number in the top row should be 3, and the bottom line should produce an error (log 0 is undefined) on your calculator. For this number just put "0".

Using the Excel chart wizard, construct survivorship curves for the 1975 and 1978 cohorts, and place them on the same set of axes. Do this by plotting the number of finches alive in each age class on a log scale (log₁₀ n_x1000) against their age or age class in years for each cohort. See Figures 6.2 and 6.3 in your textbook for examples of properly constructed survivorship curves.

Do the survivorship curves for the two cohorts differ from one another? If so, speculate on what might have caused the differences.

Compare the survivorship curves for the 1975 and 1978 cohorts to the idealized (sometimes called hypothetical) survivorship curves shown in Figure 6.4 in your textbook. Characterize the survivorship curves for each cohort as a Type I, Type II, or Type III, or some combination of these.

Examine the male and female fecundity schedules for the 1975 cohort (Table 3) and the 1978 cohort (Table 4). In words, characterize the age-based fertility (m_x) of males and females for each cohort. Do the fertility schedules for the two cohorts differ from one another? If so, speculate on what might have caused the differences.

Using the column marked l_xm_x in Tables 3 and 4, calculate the net reproductive rate, R₀), for males and females separately for both the 1975 and 1978 cohorts. Do this by multiplying the survivorship (l_x) by the age-specific fecundity (m_x) for each individual age class and then summing those values over all age classes. See section 6.2 in your textbook for an example of this calculation. In each case, is R₀ greater than, less than, or equal to one? What do these values of R₀ indicate about the future size of G. fortis populations on Daphne Major? Why would R₀ be different for males and females?

10.

Referring to the life tables and fertility schedules, summarize the overall effects of the 1977 drought on Isla Daphne Major on the demography of G. fortis. Did the drought differential affect mortality rates (q_x) of the two cohorts? How about the mean expectation for further life (e_x)?

11.

How does the life table of the Medium Ground Finch on Daphne Major compare to that of the American Robin (see Table 6.2 in your textbook)? Compare and contrast each life table variable for these two species. Go to the library and locate the journal article containing the life table on the American Robin (D. S. Farner, 1945. Age groups and longevity in the American Robin. Wilson Bulletin 57: 56–74). After reading the article, speculate about the differing environmental pressures on Darwin’s Medium Ground Finch and the American Robin.

12.

How do the survivorship curves for the Medium Ground Finch compare to that of the American Robin (see Figure 6.3 in your textbook)? Characterize the survivorship curves for each species as Type I, Type II, or Type III. See Figure 6.4 in your textbook for an illustration of the idealized survivorship curves. Describe the characteristics of species with each type of survivorship curve.

13.

Speculate about the nature of the age-based mortality (i.e., would all age classes experience the same mortality rates?) in the Geospiza fortis population on Isla Daphne Major given the following scenarios:

A major El Niño event takes place next year.
The West Nile Virus invades Isla Daphne Major from the mainland and infects the Medium Ground Finch population.
Environmental problems get worse because of the increasing numbers of ecotourists to the Galapagos Islands and pollution-related diseases in G. fortis increases.